The present invention relates to a method for producing an ethylene-vinyl acetate copolymer aqueous emulsion. More specifically, the present invention relates to a method for producing an ethylene-vinyl acetate copolymer aqueous emulsion which uses a redox catalyst in which the reducing agent comprises a transition metal salt and an erysorbic acid and/or an ascorbic acid and the oxidizing agent comprises a specific amount of hydrogen peroxide.
Conventionally, an ethylene-vinyl acetate copolymer aqueous emulsion has been produced by using a redox catalyst consisting of an oxidizing agent and a reducing agent. For example, as the reducing agent, a transition metal and formaldehyde-sodium bisulfite are often used. As the oxidizing agent to be combined with the reducing agent, sodium persulfate, hydrogen peroxide, t-butyl hydro peroxide and the like are frequently used.
However, when formaldehyde-sodium bisulfite and the like are used as a reducing agent component, formalin which is said to be the main cause of sick house syndrome is generated from the resulted aqueous emulsion, meaning an undesirable phenomenon from the hygienic standpoint.
On the other hand, when erysorbic acids, ascorbic acids or the like is used as a reducing agent component instead of formaldehyde-sodium bisulfite and the like for solving the above-mentioned problem, the resulted emulsion may be discolored in some cases.
The present inventors have intensively studied for solving the above-mentioned problem when using erysorbic acid, ascorbic acids and the like as a reducing agent component. As the result, they have found that discoloration of the resulted emulsion can be suppressed by using a specific oxidizing agent, hydrogen peroxide, as the oxidizing agent to be combined, in a specific amount of 1.8 to 10 mol per 1 mol of the reducing agent component. Based on this finding, the present invention was completed.
The present invention provides a practically excellent method for producing an ethylene-vinyl acetate copolymer aqueous emulsion which uses a redox catalyst comprising a transition metal salt and an erysorbic acid and/or an ascorbic acid, as the reducing agent thereof, and 1.8 to 10 mol of hydrogen peroxide, as an oxidizing agent thereof, per one mol of the erysorbic acid and the ascorbic acid.
The present invention is characterized in that the above described specific redox catalyst is used. The specific redox catalyst comprises a reducing agent and an oxidizing agent, and the reducing agent comprises a transition metal salt and an erysorbic acid and/or an ascorbic acid, and the oxidizing agent comprises hydrogen peroxide.
Examples of the reducing agent components, erysorbic acids and ascorbic acids, include erysorbic acid, ascorbic acid, and alkali metal salts thereof such as sodium salts, potassium salts. Among them, sodium salts are preferable, and particularly, sodium erysorbate is preferably used.
The reducing agent component is usually used in an amount of 0.02 to 1.0% by weight based on vinyl acetate. When the amount is less than 0.02 wt %, the reaction may not occur in some cases. When the amount exceeds 1.0 wt %, the produced emulsion may be colored, and the performances of the resulted emulsion, for example, the strength, water-resistance and the like of a film made of this emulsion, may deteriorate, meaning undesirable phenomena.
Examples of the transition metal salt include chlorides, sulfates and the like of iron, copper, cobalt, titanium, nickel, chromium, zinc, manganese, vanadium, molybdenum, cerium and the like. Among them, ferrous chloride, ferrous sulfate, copper sulfate and the like are preferably used.
The transition metal salt is used usually in an amount of about 0.001 to 0.05 time by weight based on the reducing agent component, erysorbic acids and ascorbic acids. When the amount is less than 0.001 time by weight, the reaction may not occur. When the amount exceeds 0.05 time by weight, the produced emulsion may be colored, meaning an undesirable phenomenon.
In the present invention, hydrogen peroxide as the oxidizing agent is used in an amount of 1.8 to 10 mol per one mol of the total amount of erysorbic acids and ascorbic acids. This amount is preferably 2 to 7 mol per one mol of the total amount of erysorbic acids and ascorbic acids. When the mount is less than 1.8 mol, the resulted emulsion is discolored. When the amout exceeds 10 mol, the performances of the resulted emulsion, for example, the strength, water-resistance and the like of a film made of the emulsion, deteriorate.
Monomers which can be copolymerized with ethylene and vinyl acetate may also be used, as the raw material monomer, in addition to ethylene and vinyl acetate. Examples of the monomers which can be copolymerized with them include vinyl esters such as vinyl propionate, vinyl butyrate, vinyl pivalate, vinyl laurate, vinyl isononanoate and vinyl versatate, halogenated vinyls such as vinyl chloride and vinyl bromide, (meth)acrylates such as 2-ethylhexyl (meth)acrylate, n-butyl (meth)acrylate, ethyl (meth)acrylate and methyl (meth)acrylate, maleates, crotonates, itaconates, and mixtures thereof.
Further, monomers having a functional group such as (meth)acrylic acid, crotonic acid, maleic acid, itaconic acid, acrylamide, N-methylolacrylamide, N-butoxymethylacrylamide, 2-hydroxyethyl methacrylate, glycidyl methacrylate, allyl sulfonate, vinyl sulfonate, triallyl cyanurate, triallyl isocyanurate, diallyl phthalate, and salts thereof, and the like can be used as the monomer which can be copolymerized with them.
The ratio by weight of an ethylene unit to a vinyl acetate unit in the copolymer is usually from about 5/95 to 40/60. The effects of the present invention are realized particularly in the copolymer in which the ratio by weight of an ethylene unit to a vinyl acetate unit is from about 10/90 to 30/70.
When, a copolymerizable monomer unit is contained, it is usually contained in an amount of 0.1 to 10 times by weight based on the vinyl acetate unit.
For producing an ethylene-vinyl acetate copolymer aqueous emulsion by co-polymerizing the above monomers, an emulsion polymerization method is adopted. As an example of the method, following method is mentioned.
A vinyl acetate monomer, a copolymerizable monomer if used, a transition metal salt, and a partially saponified polyvinyl alcohol and the like as an emulsifier are charged in a pressure proof reaction vessel. After heating to about 40-70xc2x0 C., the reaction system is purged with ethylene to remove oxygen, then ethylene is further introduced to provide an inner pressure of about 1-7 Mpa. Thereafter, hydrogen peroxide and an aqueous solution of an erysorbic acid and/or an ascorbic acid, reducing agent components, are dropped sequentially to conduct an emulsion polymerization.
An organic peroxide can also be added in a small amount in the latter period of polymerization wherein the concentration of unreacted monomers have reached a value as low as several %. By this operation, the concentration of unreacted monomers can be reduced to several thousands ppm or lower in a relatively short period of time. Examples of the organic peroxide include tertial butyl perbenzoate, dilauryl peroxide, benzoyl peroxide and tertial butyl hydroperoxide. Such an organic peroxide is generally used in an amount of 0.0001-0.005 time by weight based on the weight of vinyl acetate.
The emulsifier used in the present invention is not particularly restricted. Examples thereof include polyvinyl alcohol, protective colloid such as hydroxyethylcellulose, methylcellulose and carboxymethylcellulose, nonionic surfactants such as a polyoxyethylene alkyl ether, polyoxyethylene alkylphenol ether, polyoxyethylenepolyoxypropylene block copolymer, polyoxyethylene fatty ester and polyoxyethylenesorbitan fatty ester, and anionic surfactants such as an alkyl sulfate, alkylbenzenesulfonate, alkylsulfosuccinate, alkyldiphenyl ether disulfonate, polyoxyethylenealkylsulfate and polyoxyethylenealkylphosphate.